Digital video tape recorder for digital HDTV

ABSTRACT

A digital video tape record/playback apparatus (FIG.  10 ) for processing a high definition television signal includes provision for controlling the occurrence of high priority (HP) image information. Such information is selectively recorded at normal speed in tape tracks within predetermined regions (HP 1 -HP 5 ) so as to appear along a tape head scanning path associate with predetermined tape playback speeds greater than normal (e.g., 20× normal). Input high priority data cells may be reordered or duplicated for recording in the predetermined tape regions in other than received order. In either case, the input datastream is massaged to produce a viewable image in a fast playback mode. In an illustrated system for receiving an MPEG coded datastream, an output datastream at greater than normal playback speed comprises either intracoded I-frame data cells only, or a whole Group of Pictures (GOP).

FIELD OF THE INVENTION

This invention is related to the field of digital video signalprocessing, and more particularly to processing a digital highdefinitition television signal by a digital video taperecording/reproducing system.

BACKGROUND OF THE INVENTION

Digital high definition television (HDTV) systems have recently beenproposed. One such system, proposed by the Advanced Television ResearchConsortium and generally known as the AD-HDTV system, prioritizes anddigitally processes a high definition television signal subjected toMPEG-like variable length coding. MPEG is a standardized coding formatbeing established by the International Organization for Standardization.The standard is described in the document “Internation Organization forStandardization,” ISO/IEC DIS 11172, CD 11172-1, CD 11172-2, CD 11172-3Rev. 1, version of Jan. 21, 1992, Coding for Motion Pictures andAssociated Audio for Digital Storage Media, which document isincorporated herein by reference for description of the general codeformat. Aspects of the AD-HDTV system are described in U.S. Pat. No.5,168,356-Acampora et al. In the system described by Acampora, codewordsare prioritized to reflect high priority and relatively lower prioritystandard priority information in a digital datastream. Codewords areformed into transport packets, or cells. Each transport packet includesa packed data payload section prefaced by a header which containsinformation identifying the associated payload data.

It is desirable to record and reproduce (playback) such a digital HDTVsignal by means of a device such as a consumer video cassette recorder(VCR) for example. Such a device uses two or more magnetic heads mountedon the periphery of a rotating drum. The heads are physically separatedby a predetermined amount, and record/reproduce a signal in alternatesuccessive angled tracks on a magnetic tape. Both consumer andprofessional VCRs often include provision for special “trick” featuressuch as variable speed forward (eg., fast search), reverse andfreeze-frame. The disclosed recording/reproducing apparatus is capableof reproducing pre-recorded media, and can record both a receivedbroadcast high definition television such as a signal in accordance withthe AD-HDTV format, as well as live pictures from a camera. The presentinvention is directed to means for facilitating the operation of highdefinition digital video recorder/reproducing apparatus, with respect tospecial features operation in particular.

SUMMARY OF THE INVENTION

In accordance with the principles of the present invention, digitalvideo tape recording/reproducing apparatus suitable for processing ahigh definition television signal includes provision for controlling theoccurrence of high priority image information. Such information isselectively recorded at normal speed at predetermined regions on tapetracks so as to be appear along a tape head scanning path associatedwith specific reproducing speeds greater than normal (eg., 20× normal).Input high priority data may be reordered or duplicated for recording inpredetermined tape regions. In either case, input data is manipulated toproduce a viewable image during reproduction at higher than nominalreproduction speeds, eg., in a fast search mode. In an illustratedsystem for receiving an MPEG coded datastream, an output datastream atgreater than normal reproduction speed comprises either intracodedI-frame data cells only, or a whole Groun of Pictures (GOP).

In accordance with a feature of the disclosed system, featurebits/indicators containing operating instructions for subsequentreceiver circuits are added to the datastream.

In accordance with another feature of the disclosed system, informationis processed at the MPEG codeword level. An input MPEG coded datastreamis decoded to MPEG words, then selected MPEG words are re-coded andrecorded in tape packets.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawing:

FIG. 1 illustrates a portion of a magnetic tape showing a helical taperecording format.

FIG. 2 illustrates the head scanning path of magnetic tape tracks duringnormal play.

FIG. 3 illustrates the head scanning path of magnetic tape tracks infast scan modes.

FIG. 4 illustrates data recovery areas in fast tape scan modes.

FIGS. 5 and 6 illustrate the recorded position of high priority (HP)information for fast scan reproduction in accordance with the presentinvention.

FIG. 7 illustrates the relationship between picture, datastream andmagnetic tape information in a fast forward features mode, in thecontext of FIG. 6.

FIG. 7a is a block diagram generally illustrating functional aspects ofthe fast forward features mode described in connection with FIGS. 6 and7.

FIG. 8 is a representation of datablock groups consistent with the MPEGstandard.

FIG. 9 is a generalized pictorial representation of a hierarchical dataformat consistent with the MPEG standard.

FIGS. 10, 11 and 12 show functional details, in block diagram form, of acell-level video tape recording/reproducing system employing theinvention.

FIGS. 13-16 illustrate aspects of data cells processed by taperecording/reproducing apparatus according to the invention.

FIG. 17 shows details of the operating structure of a portion of thesystem shown in FIG. 12.

FIG. 18 shows details of a known transport block (packet) headercomponent for the AD-HDTV system.

FIG. 19 illustrates the relationship between high priority (HP) andlow/standard priority (SP) data channels in an HDTV system, and apicture frame sequence according to the MPEG standard.

FIG. 20 is a table illustrating aspects of a slow features operatingmode of a tape recording/reproducing system according to the invention.

FIG. 21 is a pictorial helpful in understanding the development ofoutput frame data in fast forward reproducing mode.

FIGS. 22 and 23 show functional details, in block diagram form, of anMPEG-level video tape recording/reproducing system employing theinvention.

FIG. 24 illustrates the general configuration of a tape packet locallygenerated by the recording/reproducing system.

DETAILED DESCRIPTION OF THE DRAWING

By way of background, and as an aid to understanding the operation ofthe invention, FIG. 1 shows a segment of magnetic tape recorded inconventional helical format. Helical-scan tape tracks are oriented at anangle as shown, and are alternatingly recorded at different azimuths,“a” and “b,” in accordance with conventional practice. Also inaccordance with conventional practice, longitudinal tracks disposedalong an outer edge of the tape contain servo and timing information.

In normal VHS television format, each track encompasses one image fieldinterval. In the case of the tape wrapping around the rotating drum 180degrees, and with two recording/playback heads, the drum completes onerevolution in two fields ({fraction (1/30)} second) at a drum speed of1800 rpm. To obtain more bandwidth for digital recording, eachfield/frame may be segmented into five portions, for example. Suchsegmentation does not present a problem since data headers provide areliable means of identifying data segments to be recombined into animage. In such a digital system the drum spins five times as fast (9000rpm). This speed is assumed to be acceptable for accomodating a 35 Mbps(bits per second) gross data rate, or about 24 Mbps net video rate, eg.,after error correction, control and audio data). Bandwidth. capacity maybe further increased by increasing the number of tracks and associatedheads. At 35 Mbps, each track contains 115 Kilobits or 14.5 KiloBytes.This amount of data is equivalent to about 100 transport packets in theAD-HDTV system. In a digital system, each track may contain more or lessthan a field/frame of information due to the nature of the variablelength coded data associated with each transport packet.

Head gaps are skewed with respect to the direction of tape head motionby a few degrees, with alternate heads tilted in opposite direction, asis known (azimuth recording/reproducing). When recording, the tracksoverlap so that the tape head is completely covered with abuttingtracks. When reproducing at a normal speed, each head is centered on itsassociated track with the correct tilt and adjacent track signals aregreatly attenuated by the effect of the azimuth and are thus effectivelydisregarded. This normal mode process is illustrated by FIG. 2, showingthe paths taken by first and second heads over respective (white andshaded) tracks during successive passes of the respective heads.

In a special “features” mode, such as fast search, each head passesacross the tape at an angle to the recorded tracks. This process isillustrated in FIG. 3 for fast-search speeds 2×, 8× and 20× faster thannormal. In effect, each head drifts across the tape tracks from one edgeto the other at a rate that is a function of the tape speed. In thehigher speed modes, data recovered by each head appears in the form of aburst of good data cells followed by a gap as the head passes over atrack section recorded with the other azimuth angle, then a burst ofgood cells from somewhere else in the picture. In this regard it isnoted that the likelihood of picture data continuity diminishes withincreasing tape speed due to the increasing number of tracks crossed asthe head transits the tape from edge to edge. The data disruption (gaps)produced by high speed tape operation is illustrated by FIG. 4. The datagaps are caused by the tape head skipping tracks rather than following agiven track continuously as in the normal speed mode, coupled with theinability of a given head intended to operate with respect to a trackrecorded at one azimuth (shaded track) to recover data from an adjacent(white) track recorded at a different azimuth.

The fast search speed range is dictated by the ability to search thetape rapidly while obtaining a reasonable preview of a displayedpicture. A speed range from about 10× to 30× normal generally satisfiesthis objective. At slower speeds, a viewer may become impatient. Thehigh end of this speed range may be too fast for skipping televisioncommercials, for example, but would still be useful for searching atape.

Fast features are distinguished by the fact that the head cannot coverall of the tape area, whereby only a portion of the available databecomes available for display. In accordance with the principles of thepresent invention, it is recognized that this deficiency may besignificantly compensated for by managing the data so as to determinewhich data subset on the tape becomes available for display at one ormore predetermined tape speeds. To this end certain data cells aremassaged, eg., duplicated or re-ordered, to permit reproduction from thetape in a desired temporal sequence at predetermined tape speeds.Specifically, high priority data are positioned on the tape so as to bereproducibly scanned by the head at selected tape speeds. This processis facilitated by the packetized datastream, and in particular byindentifying and timing information contained in the headers associatedwith the data packets, as will be explained in connection withsubsequent Figures. In the AD-HDTV system HP cells contain the mostimportant information required to reproduce the picture. The HP cellsare treated in a manner which minimizes the chances of the informationbeing lost. In over-the-air transmission the HP information is sent at ahigher power level than the LP information. For tape recording, it maybe advantageous to record the HP cells more than once.

Since at high tape scanning speeds the reproduced/displayed informationis otherwise unpredictable, in the disclosed system certain highpriority information which is known to produce a good image is recordedat specific track locations which are traversed by the head at selectedtape speeds. This high priority information includes low videofrequencies including DC, important AC image information, audio andsynchronizing information, for example. This recorded data format isillustrated by FIG. 6, which is derived from FIG. 5.

One function of the HP information is synchronization of the datapresentation with the arrival of data in the bitstream. The video inputto the system arrives as pictures or frames that occur at constant rate.The encoding process produces a variable length coded bitstream in whichindividual frames occupy different numbers of bits, and hence differenttime intervals. The receiver must produce video output frames atconstant rate. In addition, the decoder must have all of the input bitsrequired to generate an output frame at the time that the bits areneeded. In the AD-HDTV system, timing packets in the HP channel areused, in conjunction with the picture header information in the bitstream to achieve the desired synchronization. The picture headerinformation in the bit stream occurs in the HP data cells at thebeginning of the data for each frame. These picture headers include anindentifier for the picture sequence number. This picture headersequence number is, in effect, a count, going from 0 to 1023 of framesin the video sequence. Picture (or frame) 123 follows picture 122 and isfollowed by picture 124. The picture header information can be thoughtof as conveying the meaning: “The following bits determine how to makeframe 123.” The timing cells in the HP channel are inserted by theencoder at approximately a constant rate, the display frame rate. Thetiming cells do not align with the picture data. When each timing cellarrives, it is time for the decoder to make an output picture. Thetiming cells contain a sequence number that corresponds to the picturethat is to be made. At some time, a timing cell arrives, containing asequence number 122, indicating that frame 122 is to be made. Verynearly one frame time later, a timing cell arrives containing sequencenumber 123, indicating that frame 123 is to be made. The time of arrivalof the timing cell indicates when the frame is to be generated, and thecontents of the timing cell indicates which frame is to be made. If thedata for frame 123 arrives before the timing cell for frame 123, thedecoder waits for the proper timing cell. If the timing cell for cell123 arrives at the decoder and the buffer in the decoder containssomething other than the picture header for cell 123, the data in thebuffer is discarded. This recorded data format is illustrated by FIG. 6,which is derived from FIG. 5. The timing cell information can be thoughtof as conveying the meaning: “Now is the time to make frame 123.”

Additional redundant information is carried in the timing cell, such asthe type of frame (I,B or P). The timing cell is also the vehicle fortransmitting the decoder control bits or feature bits described below.

As shown in FIG. 6, high priority (HP) data packets (cells) are recordedat five predetermined regions of a track, designated as HP1 through HP5.The track sections passed by the head during non-standard speed playback(eg., fast search) determine the recorded locations of HP data. HP datais spatially recorded along the track to facilitate recovery duringother than nominal reproduction speeds when the head traverses multiplerecorded tracks during a single pass.

The HP data packets are located so that they encompass all of the tapetrack segments within each designated HP region, not only the blackareas within each HP region. The black areas indicate the head contactareas along the head path at predetermined 2×, 8× and 20× normal tapespeeds (see FIG. 3), ie., areas where the head acquires information fromthe tape at these predetermined speeds. The remaining interveningregions containing head contact areas are considered as low priority(LP) regions and are designated as regions LP1 through LP3 for the 20×normal speed example. Other low priority regions, eg., between regionsHP2, HP3 and between regions HP4, HP5 have not been labelled as such.Low priority data is written to the tape as it appears in the originalinput datastream, ie., without re-ordering.

The following discussion is made with respect to tape speed at 20×normal speed. In the AD-HDTV system, HP information includes lowfrequency video information and audio and synchronizing (timing)information. In the present embodiment, the HP regions are filled withre-ordered intracoded I-frame cell data, and the LP regions arbitrarilycontain B frame or P frame cell data among other low priorityinformation as received. It is noted that the HP regions are selected sothat good results are produced at more than one speed, ie., HP data ispredicatably recovered at these speeds. Specifically, each of the threeillustrated tape speeds (2×8× and 20× normal) will have HP informationrecorded to coincide with the associated head path so that a viewable orrecognizable image will be produced by a head following each of theassociated paths. For example, the uppermost 20× speed path includesfive designated HP regions (including one at the origin), each of whichhas a counterpart at the other designated high tape speeds. In FIG. 6,the numbers shown in the third horizontal track from the top designatethe nominal width of the associated HP or LP region, in data slices.Thus, for example, region HP1 contains 2 high priority data slices,region LP1 contains 6 low priority data slices, region HP2 comprises 3data slices, and so on. It is to be understood that these slicedesignations are a simplication in the interest of providing a clearexplanation. The HP data is recorded at normal tape speed so that thisdata is traversed by the head at the selected fast forward featuresspeeds, as will be discussed inconnection with FIG. 7.

In the recording process, the variable-length-encoded bitstream is putonto the tape. An example not involving duplication of data will bedescribed first. The total bit rate to be recorded corresponds to thetotal area of tape to be recorded. In previous recording methods, thedata to be recorded is put on the tape sequentially. In the currentexample, there is no direct correspondence of the position of the dataon the tape to the time sequence of the video. The bitstream data isrearranged, cell by cell, to provide the feature mode playback. Adesired pattern of cells on the tracks is pre-determined, based on thedesired playback feature speeds. As shown in FIG. 6, the planned patternis: 2 HP cells, 6 SP cells, 3 HP cells, 1 SP cell, 4 HP cells, 5 SPcells, 4 HP cells, 2 SP cells, 3 HP cells and 12 LP cells. Thecorresponding sequential regions in FIG. 6 are marked HP, LP1, HP2, (LPnot-marked), HP3, LP2, HP4, (LP not-marked), HP5, and LP3. The taperegions are filled with the next available cell of the correspondingtype that is available in the prioritized bitstreams. In normalplayback, the full data stream is recovered from the tape. The first 2cells of a track go into the HP output stream, the next 6 cells go intothe LP stream, the next 3 into HP, and so forth.

Again, with reference to FIG. 6, now assume that the same tape is playedback at 2× speed. Valid data is read form the tape in the region markedin black. The reading process starts on track 1. The first 2 cells readare assigned to HP, the next 6 cells assigned to SP, the next 3 to HP,the next cell to SP and so forth. Somewhere in region LP3 the databecomes invalid. Valid data resumes with the other azimuth head on track4, and repeats the same pattern. It can be seen that not all of the datais recovered, but the data that was lost was LP data. It is acharacteristic of the prioritization of AD-HDTV that pictures can begenerated from HP data only.

FIG. 7 illustrates the process of tape machine operation in a fastforward features mode at 20× normal speed, with respect to cell datareordered into a desired sequence of HP and LP data regions as shown inFIG. 6. For the purpose of the following example it is assumed that thereordered data placed in HP regions HP1 through HP5 is MPEG intraframecoded I-frame data, which is temporally coherent information independentof other information, unlike B-frame or P-frame data which arepredictively coded. To simplify the following discussion of FIGS. 6 and7, it is assumed that low priority LP regions LP1, LP2 and LP3respectively contain B₁, B₂ and P₁ frame data in that order, althoughthe actual content of the LP regions would be arbitrary, ie., the LPdata would occur in the LP regions in the order received in thedatastream.

To help understand this material, reference is made to FIGS. 8 and 9together with pictorial (A) of FIG. 7, showing the MPEG arrangement of aGroup of Pictures (GOP). A GOP, in the context of the MPEG standard,contains 9 image frames, beginning with an I-frame followed by asequence of B- and P-frames as follows: I, B₁, B₂, P₁, B₃, B₄, P₂, B₅,B₆. An I-frame usually requires many more bits than either B or Pframes. A GOP exhibits a display time of {fraction (9/30)} sec., orabout 0.3 seconds. At a channel rate of 24 Mbps, an average GOPcorresponds to 7.2 Mb in size (24×{fraction (9/30)}). A GOP may vary insize from 1.2 Mb to 13.2 Mb. An AD-HDTV receiver typically will containtwo stored past and future I and P anchor frames. When a B frame isreceived, an output image frame will be generated from the stored anchorframes. When a non-B frame is received, the existing future anchor frameis moved to the position of the past anchor frame, and a new futureanchor frame is created from the received data while the past anchorframe is being displayed.

More specifically, in this example the input datastream comprises adata-compressed sequence of frames that are coded compatible with theMPEG format. This format is hierarchical and is illustrated inabbreviated form in FIG. 9. The MPEG hierarchical format includes aplurality of layers each with respective header information. Nominallyeach header includes a start code, data related to the respective layerand provision for adding header extensions. Each header includesinformation related to the data in the data packet with which the headeris associated. The header information aids data assembly andsynchronization eg., at a receiver, and includes information such asservice type (eg., audio, video), frame type, frame number and slicenumber, for example. A header of this type and its processing aredescribed in the context of an AD-HDTV signal processing systememploying MPEG signal coding in previously mentioned U.S. Pat. No.5,168,356-Acampora et al. FIG. 18 of the present application showsdetails of the transport block header used in the AD-HDTV systemdescribed in the Acampora et al. patent. FIG. 19 shows the timingrelationship between the AD-HDTV high priority (HP) and the relativelylower priority standard priority (SP) datastreams, and the time sequenceof the MPEG coded I, B and P frames.

When referring to the MPEG compatible signal processed by the system,what is meant is that (a) successive picture fields/frames of videosignals are encoded according to an I, P, B coding sequence, and (b)coded data at the picture level is encoded in MPEG compatible slices orgroup of blocks, where the number of slices per field/frame may differand the number of macro blocks per slice may differ. An I coded frame isone which is intraframe compressed (coded) such that only I framecompressed data is required to reproduce an image. P coded frames arecoded according to a forward motion compensated predictive method, wherethe P frame coded data is generated from the current frame and an I or Pframe occurring before the current frame. B coded frames are codedaccording to a bidirectionally motion compensated predictive method. TheB coded frame data is generated from the current frame and from I and Pframes occurring both before and after the current frame.

The coded signal processed by the present system is segmented intogroups of pictures (frames), or GOPs, illustrated by the row of boxes L2(FIG. 9). Each GOP (L2) includes a header followed by picture datasegments in the form of a sequence of nine picture frames I, B, B, P, B,B, P, B, B as illustrated by pictorial (A) of FIG. 7. The GOP headerincludes data related to the horizontal and vertical picture size, theaspect ratio, the field/frame rate, the bit rate and other information.

The picture data (L3) corresponding to respective picture framesincludes a picture header followed by slice data (L4). The pictureheader includes a field/frame number and a picture code type. Each slice(L4) includes a slice header followed by a plurality of blocks of dataMBi. The slice header includes a group number and a quantizationparameter.

Each block MBi (L5) represents a macroblock and includes a headerfollowed by motion vectors and coded coefficients. The MBi headersinclude a macroblock address, a macroblock type and a quantizationparameter. The coded coefficients are illustrated in layer L6. Eachmacroblock includes 6 blocks, including four luminance blocks, one Uchrominance block and one V chrominance block, as shown in FIG. 8. Ablock represents a matrix of pixels, e.g., 8×8, over which a discretecosine transform (DCT) is performed in this example. The four luminanceblocks are a 2×2 matrix of contiguous luminance blocks representing,e.g., a 16×16 pixel matrix. The chrominance (U and V) blocks representthe same total area as the four luminance blocks. That is, beforecompression the chrominance signal is subsampled by a factor of twohorizontally and vertically relative to luminance. A slice of datacorresponds to data representing a rectangular portion of an imagecorresponding to an area represented by a contiguous group ofmacroblocks. A frame may include a raster scan of 360 slices, 60 slicesvertically by 6 slices horizontally. The block coefficients are DCTtransformed one block at a time. The DC coefficient occurs first,followed by respective DCT AC coefficients in the order of theirrelative importance. An end-of-block code EOB is appended at the end ofeach successively occurring block of data.

Referring again to FIG. 7, pictorial (B) is a simplified expanded viewof pictorial (A). Each I, B and P frame comprises image slices with adata component and a header component which defines the associated slicedata component. Pictorial (C) represents the data recovered from themagnetic tape at 20× normal speed. Each segment of recorded data (C)represents a burst of about 6-7 data cells followed by a gap of about3-4 data cells, at 20× normal speed. The recovered data is derived froma tape with data arranged into predetermined HP/LP regions as shown inFIG. 6. In the interest of clarity, pictorial (C) does not show the datagaps resulting as the head passes over other azimuth recorded tracks.Pictorial (D) illustrates the datastream at the output of the tapemachine when playing back at 20× normal speed. The playback data (D) hasbeen re-ordered and converted (from recorded data C) to the standardMPEG sequence of I, B, B, P . . . frames which a receiver's signalprocessing circuits expect to see for display processing purposes. Thuspictorial (D) represents image data in a fast forward features mode at20× normal speed based upon an arrangement of the tape into HP and LPsegments as shown in FIG. 6.

The tape data packets are reordered into prioritized regions as shownand discussed with respect to FIG. 6 by using information includingtiming information contained in the tape packet headers together with anindexing system as will be described. The recovered tape data sequencebegins with I-frame information since this information begins a GOPaccording to the MPEG standard and is therefore readily identified. Inaddition, a Picture Start codeword is associated with the beginning of aGOP. Beginning with track 1 at 20× speed, the first recovered data fromhigh priority region BP1 is I frame slice information which in pictorial(C) is shown as I1. The next region scanned by the tape head along the20× speed path is a low priority region LP1 occurring at track 3. Datarecovered from this region, assumed to be B frame data, is shown as B₃.The third region scanned by the tape head at 20× speed is high priorityregion HP3 at track 5. Data recovered from this region is shown as I5 inpictorial C This process continues by acquiring I frame data from tracks7, 11 and 13, B frame data from track 9, and P frame data from tracks15-19, in the sequence shown. In pictorial (C), the subscript associatedwith a given I, B or P frame designates the track from which the datawas recovered by the head. The data recovered from the LP3 region of thetape corresponds to disconnectet regions of LP data from that isprobably from somewhere in frames 15 through 19. Because of the natureof the variable-length-encoded bitstream, it is generally not possibleto specifically indicate which frames of video correspond to specificregions of the tape.

The recovered data in pictorial (C) is inappropriate for processing by areceiver, since a receiver expects the see data in the I B B P B B P B BMPEG frame format. Thus the data recovered at 20× normal speed(pictorial C) must be re-converted back to an MPEG compatibledatastream. Such a re-converted MPEG compatible datastream reflecting20× tape operation is illustrated by pictorial (D) and is developed asfollows. First, the tape packet headers of the recorded data inpictorial C are examined to identify the recovered information. The datais then re-ordered into the standard MPEG format by grouping I, B and Pframe data together in sequence. The HP1-HP5 I frame data is placedfirst in sequence in the output datastream (I₁, I₅, I₇, I₁₁, I₁₃ . . .), followed by LP1 and LP2 B frame data (B₃, B₉ . . . ), other B framedata, and LP3 P frame data (P₁₅₋₁₉ . . . ).

FIG. 7A generally illustrates the process by which this is accomplished.In step 70, the input I, B, P datastream (pictorial B in FIG. 7) isreordered so that the data cells exhibit the sequence of high and lowpriority regions shown in FIG. 6. In step 71 tape packet header data isprovided for identifying the reordered data. The reordered tape packetsare recorded at normal speed, and played back at 20× normal speed in afeatures mode (step 72). During 20× playback the headers of the recordedpackets are examined in step 73. These headers contain informationidentifying the I, B, P data sequence shown in pictorial (C) in FIG. 7.In step 74 the data sequence is converted back to the standard MPEGsequence expected by the receiver (pictorial D). The re-converted IBBP .. . frame sequence is stored in a multiple page (I, B, B, P) memory(step 76), which is continually scanned for deriving output data to beapplied to a tape signal receiver via a playback output processor/buffer(step 78). Addresses in frame stores which have not been altered toreflect reordered data remain unchanged, ie., with old data from priortape head scans.

At normal tape speed, all IPB frame data derives from one tape track, incontrast to the 20× normal speed situation where, for example, I framedata are obtained from more than one track. With the disclosedtechnique, reordered HP I frame data is advantageously placed so as tobe predictably recovered at several places along the tape head path, atmore than one tape speed in this example. A visual presentation in theillustrative I frame example is a series of still image information at arefresh rate likely to be about 3 to 5 frames a second. Each frame iscomposed of pieces (from macroblocks, through slices to larger pieces)from the corresponding collection interval.

The disclosed technique for remapping high priority cell data intopredetermined tape regions for reproducible recovery during a high speedfeatures mode essentially first involves determining what scan ratesproduce good results. Typically, a scan rate that is moderate (2, 3, or4×) is desired to allow accelerated viewing of pictures. In addition,the fastest rate that produces viewable pictures is desired (eg., 20×)to permit rapid skipping of unwanted scenes. Next, the areas of the tapetraversed by the head at these scan rates is mapped, ie., the head trackis determined. Finally, HP cells are assigned (re-ordered) to designatedtape areas traversed by the head at selected scan rates. A duplicate ofa timing cell associated with the beginning of a GOP may be placed atthe beginning of a tape scan, since there is a high probability ofrecovering such a cell. If HP cells do not completely fill selectedareas, the cells are distributed in clumps in the centers of theselected areas. It is noted that, during fast scans, as the pickup headis displaced with respect to the position centered with respect to thetrack (of correct azimuth), the signal quality follows a trapezoidalshape. Specifically: the signal is not present, the signal improves, thesignal is good while the head is aligned with the center of the track,the signal gets worse, and the signal is lost. The HP data is located inthe regions where the signal is good. If the selected areas are largeenough for the original number of cells, but not large enough to holdall the duplicate cells, the duplicates are distributed uniformlythroughout the area. If the selected areas are too small for theoriginal number of HP cells, the least useful scan rate should beignored and the head track is remapped at the time of product design.

With this technique, the fraction of the image which is visually good oracceptable is greater than that which would otherwise result, eventhough some temporal segmentation may be present. The amount of temporalsegmentation which may appear is a function of frame-to-frame motion,which may be very little in many cases. The illustrated tape speeds arethose which are considered to give good results, but other speeds mayproduce better results. The disclosed system advantageously offersflexibility with respect to choice of speeds in the high speed featuresoperation mode, including speeds which are non-multiples of the numberof frames in a GOP.

When presenting a picture at a fast scan speed, a decision must be madeabout when to present data to the output decoder. Recall that thevariable bit length encoded data recovered in a frame time does notcorrespond to a frame's worth of data. One frame time may containseveral B-frame's worth of data or only a fraction of an I-frame's data.The operation of an AD-HDTV decoder or an MPEG decoder takes place inunits of pictures and frames. If a frame's time data corresponding topart of an I frame is delivered to a decoder, only part of the pictureis produced. To update the entire picture with refresh action, it isnecessary to accumulate enough of the bitstream to describe how to makean entire picture. When this information is delivered to the decoder,the corresponding timing cell may be delivered to cause the generationof the frame.

In the example illustrated by FIGS. 6 and 7, it was assumed that I framedata is the high priority data arranged in the designated HP regions.Since the tape head will recover data from both high and low priorityregions at 20× normal speed, the playback data stream (FIG. 7, pictorialC) will include high priority I frame data and low priority B and Pframe data. When it is intended to display an image at 20× speedobtained only from I frame information, the tape machine will outputonly the I frame data (eg., only I₁-I₁₃ in FIG. 7 (C)). This may beeasily accomplished by examining the headers of the playback cells andrejecting all except those cells containing I frame information. Sincean I frame encompasses approximately 0.2 seconds, at 20× normal speedthe cells from 4 seconds of original video will be collected to producea 0.2 second collection interval. As noted above, the output will be aseries of I frames only in this case, which is illustrated by FIG. 21.In FIG. 21 the upper pictorial represents a portion of data recoveredfrom the tape at 20× speed, shown relative to real time in milliseconds,where only I frame cells are used. The data segments designated “not I”represent rejected low priority data. Thus the upper portion of FIG. 21corresponds to the output playback datastream shown in pictorial (C) ofFIG. 7 with I frame data cells grouped together and B, P frame datacells rejected. The lower pictorial of FIG. 21 illustrates the formationof a synthetic I frame from a composite of data acquired from 20 GOPs,shown with respect to tape speed in seconds (at 20× normal speed).Although the variable length coded bitstream does not necessarilycorrespond to equal duration pictures, there is a maximum deviation intime of the picture data in the bitstream from the time at which thetiming cell causes the generation of the output frame. This maximum timedeviation is set by the buffer size, and is typically ¼ second. If theGOP consists of 9 frames, then 20 GOPs at 20× speed are accumulated in 9frame times ({fraction (9/30)} seconds=0.3 second), and 20 GOPs of dataon the tape will have passed in 0.3+0.25=0.45 second. Null cells areinserted in the playback datastream in place of the rejected cells tomaintain datastream continuity. A visual presentation in theillustrative I frame only example is a series of still images at arefresh rate likely to be about 3 to 5 frames a second. Each frame iscomposed of pieces (from macroblocks, through slices to larger pieces)from the corresponding collection interval. Some I frame data may beused in more than one output frame.

Alternatively, a playback output image may be comprised of a compositeGOP if all recovered HP frames and SP frames are retained in the outputplayback datastream (as shown in FIG. 7 (C)). Since a GOP encompassesapproximately 0.3 seconds in real time, at 20× normal speed the cellsfrom approximately 6 seconds of original video will be collected toproduce a 0.3 second collection interval for a GOP. In this case the 20×playback output is one GOP in which each frame is a composite of the 20corresponding frames of input. At a 20× scan rate, 20 tape GOPs resultin one output GOP. For example, the second B frame (B₂) of the 20×playback output is made from the 20 2nd B frames in the sample. Anadvantage of acquiring a whole GOP in the high speed tape scanningfunction is that a picture with smoother motion may result due to thepresence of spatially coded P and B frame cells, and motion vectors fromthe P and B frames are likely to be in approximately the correctdirection. For example, consider a picture composed of a speaking personin front of a slowly panning background. In the case of fast forwardplayback using I frame information only, the still parts of the speakerwill look good, but the moving parts of the speaker will be somewhatdistorted, and the background will appear as a sequence of distortedstills. If the whole GOP is used during playback, the I frame imageportions appear about the same, but some blocks of the picture change asother frames of the GOP are displayed. The moving background is likelyto remain distorted, but much of the picture moves smoothly inapproximately the correct direction.

The implementation of whole GOP scanning uses a constant size intervalof interest, which is 9 frames in the case of the AD-HDTV system. If thedefinition of a GOP changes to other than 9 frames in another system,the programming and operation of an index-index and computer controller(shown in FIGS. 10 and 12) are adjusted accordingly. All GOP frames areused. The start of the interval of interest is the first cell of the Iframe beginning the GOP. The sequence of frame types (I, B₁, B₂, P₁, B₃,B₄, P₂, B₅, B₆) within retreived GOPs is tracked, and cells are sortedwithin like frames, eg., cells that came from B₃ frames are put into theB₃ output frame.

The process of redistributing the HP cells during recording involvesbuffering the cells into cell memory as they are received. A CellAnalysis Processor (as will be described) knows the time sequence of thecells being written to the memory. Specifically, the Cell AnalysisProcessor monitors the position of a cell in the input data streamrelative to the start of a Group of Pictures. This position is encodedin packet headers produced by the tape machine, eg., unit 1016 in FIG.10. This is accomplished without difficulty since the tape mechanism canmaintain a precise count of the tape position in terms of the followingfactors: hh (hours), mm (minutes), ss (seconds), ff (frame number), tt(track number) and pp (packet number). This information is unambiguous,and is mechanically derived from the head position and the longitudinaltiming/sync track on the tape itself. This position information can berepresented as a binary value. During the recording process, each cellis associated with a 10-bit linear temporal reference number at theframe rate. When recording, the last picture header processed containsthe temporal reference number. This number is temporally stored in therecorder, and recorded in the next tape related header to be written bythe tape mechanism. The GOP starting point and its temporal referenceare readily determined as noted earlier. As the (AD-HDTV) cells arrive,they are serially numbered within a GOP beginning at the GOP start. Thisis a 14-bit number. The difference between the linear temporal referenceand the 10 least significant bits to the tape position, along with the14-bit cell serial number, are included in the tape packet header. Anypacket, when recovered during the playback process, yields this 24-bitnumber that can be used to regenerate the exact time of arrival of thecell.

At normal speed playback, the original position of each cell in thedatastream is retreived from the tape packet headers. The playback celldatastream can then be rearranged in the original order.

A general arrangement of a high definition video taperecorder/reproducer and a high definition television receiver is shownin FIG. 10. A VTR 1008 receives an input HDTV signal which is decoded byan HDTV decoder 1010. If the input signal is of the AD-HDTV type,decoder 1010 may be arranged as described in U.S. Pat. No.5,168,356-Acampora, some aspects of which are shown in FIG. 11. Cellheader information is analyzed by unit 1012 to obtain data about cellcontents including timing and sequence, and high priority cells areduplicated by unit 1014 as an aid to implementing features modeoperation. Specifically, selected cells are duplicated in the recordedstream to allow fast scans. In this example, where the AD-HDTV FECfunction is included in the tape machine (in decoder 1010, see unit 1154in FIG. 11), a benefit is obtained that error related overhead does nothave to be stored. The extra storage that would have been used for FECbytes may now be used for redundant (duplicate) cells to make certaininformation available during a fast search mode. Additional storagecapacity beyond the minimum required for the storage of the originalbitstream may become available through the process of elimination of FECdata as described above or may become available due to technologyimprovements in the recording process. This excess capacity is used forthe redundant storage of cells to improve fast scan performance. As wasdescribed earlier, in the design process, selected scan speeds aremapped to regions on the tape in which the HP data is to be placed. Ifextra tape capacity is available, the mapping shown in FIG. 6 may bedone for additional scan speeds. For example, if an additional scanspeed were desired, and data at the new scan speed was not recovered inregions HP2 and HP4, but was recoverable in a region just to the left ofHP2 and just to the left of HP4, the HP data that is recorded in HP2 andHP4 could be redundantly recorded in the right ends of regions LP1 andLP2. The displaced LP data from LP1 and LP2 is absorbed in the extracapacity of the media. In the illustrative example described previously,HP cells 1 and 2 were recorded in region HP1, then SP cells 1,2,3,4,5,6recorded in LP1, then HP cells 3,4 in HP2, & etc. Given extra capacity,the recording pattern is: HP cells 1, 2 in HP1, LP cells 1,2,3,4,5 inLP1, then HP cells 3,4 in a new duplicate region at the end of LP1 andthe beginning of HP2, then HP cells 3,4 again in HP2. The pattern wherethe redundant cells are stored is pre-determined when the tape machineis designed. When playing back at normal speed or 2×, the redundantcells are recovered. From the position along the scan, these cells areknown to be redundant, and are discarded.

Cells are converted to tape packets in unit 1016, eg., by means of 8:14modulation whereby 8 bits words are converted to 14 bit codewords forbit rate reduction as known. Tape packets typically include a headercomponent, an associated data component and timing/synchronizinginformation. Tape packets from unit 1016 are applied to anMPEG-compatible signal input of unit 1018 which includes tape signalprocessing networks and a tape transport mechanism. A non-MPEG auxiliaryinput of unit 1018 receives encoded tape packets from an auxiliarysource 1020 (eg., a video camera) via a non-MPEG spatial informationtape encoder 1022. Tape output signals from unit 1018 are provided to aunit 1030 which adaptively converts MPEG or non-MPEG output signals to astandard RGB (Red, Green Blue) color television signal format inresponsive to a flag bit in the datastream, and to a unit 1032 whichconverts the tape packets back to the cell format of the inputdatastream. Duplicate HP cells are eliminated by unit 1034 to provide anoutput signal datastream of the format expected by the AD-HDTV decoder1044 of television receiver 1040.

Standard RGB output signals from unit 1030 are applied to one input ofan input signal selector 1046 of HDTV receiver 1040. The cell formatoutput signal from unit 1034 is decoded by HDTV decoder 1044 at an inputof receiver 1040 before being applied to another input of selector 1046.Selector 1046 provides either RGB format signals or decoded AD-HDTV cellformat signals to video, audio, sync, etc. television signal processingand reproducing/display circuits 1042 of receiver 1040.

FIG. 11 shows HDTV decoders 1010, 1044 of FIG. 10 in greater detail. Areceived input signal is detected by modem 1150, which provides anoutput signal to a de-interleaver (de-scrambler) 1152 and a Reed-Solomonforward error correcting (FEC) decoder 1154. The corrected signal isapplied to a rate buffer 1156 which receives data at a variable ratecommensurate with the requirements of subsequent decompression networksin video decoder 1160. A transport processor performs the inverse of adata packing and prioritizing operation performed at a receiver, andadditionally performs a degree of error detection in response to paritycheck bits included in the transport packets. Transport processor 1158provides output video and audio signals to video transformdecoder/decompressor 1160 and audio decoder 1162, respectively, whichprovide audio and video output signals with a cell format including dataand header components. In an AD-HDTV system as described in U.S. Pat.No. 5,168,356-Acampora, the modem provides two output signal associatedwith HP and LP channels, and the de-interleaver, FEC control and bufferfunctions are duplicated for both the HP and LP channels.

FIG. 12 shows a more detailed block diagram of a video taperecording/reproducing device capable of operating at the data celllevel. The system of FIG. 12 includes a computer controller 1222, ie., amicroprocessor, capable of making the many decisions that must be madeat a packet/cell rate. Controller 1222 interacts with a databaseincluding cell buffer memory 1232 containing data read from the tape,and the controller maintains an index 1224 of the buffer contents. Foreach cell, the index contains information pertaining to cell location inthe buffer; cell starts at the tape reference time indicated by factorshh, mm, ss, ff, tt, pp (described previously); cell error status; cellservice type (eg., audio or video) including HP/LP indicators; cellframe number; cell frame type; and information translating between thetape temporal reference and, for example, the AD-HDTV linear temporalreference. The last mentioned item can be generated at the time the tapeis written to, and indicates which timing packet's domain a data cellbelongs to. Additional indexed information may include indicators suchas an indicator indicating the validity of the next cell in sequence, aduplication indicator showing that a cell has been duplicated elsewhere,and a duplication index pointer indicating the index location of theduplicated cell.

The index may contain as many as 13,000 entries (the word size of thelargest GOP). The size of an index entry is about 80 bits, producing astorage requirement of about 128 KBytes for the index. In addition, asan aid to controller 1222 in using the index, an index-index 1226 isalso maintained. The index-index contains pointers to frame boundarieswithin the index.

More specifically, in FIG. 12, a camera input formatter 1210 converts ananalog RGB color video signal to digital form during recording. Anotherblock, not shown, performs the inverse function for playback. Taperead-write unit 1216 performs bit level modulation/demodulation duringrecording/playback, eg., using 8:14 modulation, as known, to convert aneight bit data word to a 14 bit codeword to achieve bit rate reduction.Tape motion controller 1218 contains capstan and tracking controls, andprovides controlled acceleration starts and stops. In combination withunit 1216, unit 1218 manages the time-code function. Unit 1218determines which track is being read, and responds to instructions to goto and pause at a specified track, and to begin playing at a given trackat a certain speed, for example. Tape packet format/deformat unit 1220creates and formats tape packets, creates or decodes tape packetheaders, and performs Forward Error Control and Cyclical RedundancyChecks as well as providing error indications. HDTV decoder 1212operates as described in FIGS. 10 and 11 and provides an output cellstream including data bytes as well as information such as start-of-cellinformation, error flags, and a byte clock.

Cell analysis processor 1230 generates read/write addresses and providesdata to cell memory 1232. In the recording mode, processor 1230 examinespacket headers and timing packets and temporal references such as may beprovided in the datastream of an AD-HDTV signal, for example. Processor1230 additionally maintains a GOP and frame count, a linear temporalreference for determining frame display, pointers to previous GOPs andframes, a tape packet record index indicating exactly where a packetshould be recorded, and also controls the delivery of duplicate HP cellsto packet format unit 1220. During the playback process, unit 1230 fillscell memory 1232 with data cells, scans the data stream and cell headersin particular, creates a cell index entry for each cell and maintains aGOP and frame sequence state. In addition, unit 1226 index-index entriesare created at appropriate times to designate significant occurrencessuch as GOP boundaries, frame boundaries and timing packets in thedatastream.

Cell memory 1232 is sized to accomodate a GOP, receives cells read fromanalyzer 1230 and writes output cells to packet formatter 1220 at a portspeed of about 3 MBps. Cell index memory 1224 writes entries whileanalyzer 1230 is filling cell memory 1232. Index memory 1224 comprisesthe address function of controller 1222. Index-index 1226 is associatedwith memory 1224 and contains pointers to frame boundaries within index1224, as will be seen in connection with FIG. 17.

Cell output processor 1240 generates read addresses for cell memory 1232and provides output cells from cell memory 1232 under control ofcontroller 1222 during the playback mode. A FIFO buffer in outputprocessor 1240 is loaded with addresses and instructions from controller1222. Under control of unit 1222, some data passes from memory 1232 tothe output of output processor 1240 without alteration, while other datamay be altered (eg., service type sequence counts, timing cells andtemporal reference data).

Computer controller 1222 implements various features such as FastForward, Slow Motion and Freeze Frame in response to input user controlsignals (eg., from a user interface such as a remote control unit) bycommunication with cell analysis processor 1230, cell output processor1240 and tape motion controller 1218 via flags, buffers and registers asappropriate. Controller 1222 processes about 30,000 cells/second andexhibits about 33 microseconds/decision, on average.

Following is a more detailed description of the information processed bycell memory 1232 and cell index memory 1224. Units 1224 and 1232 processtwo basic types of cells from which FEC and error detection data havealready been removed. These cells are video cells, and timing and audiocells as shown generally in FIG. 13. In these cells ST designates theService Type code portion (video, audio, timing), and H designates theHeader portion. Every cell begins with a Service Type byte. In additionto the type identification (video, audio, timing . . . ) the bytecontains a four-bit cyclical continuity count, ie., a Service TypeSequence Number. The continuity count must be cyclical within eachservice type. If not, the associated cell is assumed to be an(otherwise) undetected error, eg., a lost packet. The Sequence Numberalso indicates an incorrect service type, such as if audio and videotypes are interchanged. Timing and audio cells have no specific headerin this example. As illustrated by FIG. 14, the Service Type is a Byteindicating high priority video VH or low priority video VL. Similarly,FIG. 15 illustrates the Service Type byte indicating audio (always highpriority) AH, and a timing cell TH (always high priority). Numbering isindicated by the symbol “nn.” In video cells, the header contains a5-bit frame number for aligning datastreams. In the AD-HDTV system,there is one HP cell for every four LP cells. Usually the HP cell isvideo, occasionally. the HP cell is audio, and once a frame the HP cellis a timing cell.

There are four types of video headers, as shown in FIG. 16. VH and VLdesignate high and low priority video components as discussed. B, P andI designate the frame components of a GOP. F designates a special case Iframe, which is a high priority I frame containing the beginning of aframe at Slice 0. This cell starts with a Picture Start Code (PSC),which is followed by a temporal reference for the new frame. The symbolff indicates the frame number. In the playback mode, cell memory unit1232 is loaded as a ring buffer. Somewhere in the memory is thebeginning of a GOP. This GOP boundary is defined as the first HP or SPcell designated by Frame Type F.

Temporal reference information, eg., 10-bit information, is sent via thedatastream timing packets to establish a flywheeled local lineartemporal reference which determines which frame to display and when. TheTemporal Reference also appears in the HP data following an MPEG PictureStart codeword, and can be recovered as the 10 bits immediatelyfollowing the Picture Start codeword in a frame type “0” HP cell.

FIG. 17 illustrates the process by which cell memory data is accessed inthe system of FIG. 12. While putting cells into memory 1232, cellanalyzer 1230 has also filled in the cell memory index of unit 1224.When cell analyzer 1230 sees a GOP start, or other significantinformation, it also fills in entries in Index-index 1226. For example,when a GOP start is present, a register 1223 in computer controller 1222indicates that location (address) 41 of Index-index 1226 is a GOP start.Register address 41 indicates that this GOP start (GS) is high priority(H) and that its index location (address) is 3210. That index locationcontains an entry indicating that the corresponding tape location (“i”)is 4 minutes, 7 seconds, 9 fields, 2 tracks, 0 packets, and that thetemporal reference (“t”) is 222, for example. Flags (“f”) may indicatethat there are no errors, that the next cell is also good, etc. Finally,the index location indicates that the actual data for the cell islocated at address “12345” in the cell memory. The next Index-indexentry indicates that the entry for the Group Start, low priorityinformation (GS, L) is at index location 3333. The following Index-indexentry indicates that the entry for the timing cell for this Group Start(GS, T) is at index location 3456. The next entry is for a low priorityB frame (B, L).

Another approach to implementing features functions is recording at thelevel of MPEG codewords. In this example, this approach is based upondecoding an input AD-HDTV datastream to the level of a single stream ofMPEG codewords. Specifically, it is herein recognized that in theinterest of efficiency, only useful spatial information should beused/duplicated. Spatial and non-spatial information can be separatedmuch more easily in an MPEG codeword datastream than in an AD-HDTVdatastream. MPEG-level processing advantageously uses spatial slices,and slices containing significant numbers of spatial macroblocks resultin smoother scan presentations.

Therefore, according to this approach, an input digital datastream isdecoded to a datastream of MPEG codewords. These codewords are thenseparated into codewords representing spatial and non-spatialinformation, and certain separated spatial codewords are placed intotape data packets. The tape data packets are replicated as required, andthe spatial packets are recorded where a scanning head will recover thespatial information at tape speeds greater than normal, such as fastsearch speeds. Audio information is separated from video and processedseparately, and is applied to a dedicated audio input of an HDTVreceiver.

A general arrangement of a high definition video taperecording/reproducing device using this approach is shown in FIG. 22.Elements 2210, 2218, 2220, 2222, 2230 and 2234 are similar to units1010, 1018, 1020, 1022, 1030 and 1034, respectively, of FIG. 10. In FIG.22, HP and lower priority SP video information at a video output ofAD-HDTV decoder 2210 is stored in an elastic buffer 2211 before beingvariable length decoded into MPEG codewords and merged into a singledatastream of MPEG words in a unit 2212. The MPEG word datastream isseparated into spatial and non-spatial data by a separator 2213, whichaccomplishes this by examining codeword identifiers. Non-spatialcodewords are applied directly to a variable length coder 2214 (ie., adata compressor) to facilitate the subsequent recording process in unit2218. Spatial codewords are duplicated by a unit 2215 before being VLCcoded by unit 2214. Only important spatial data is duplicated by unit2215, since there is a large amount of spatial data. For example, acomplicated still picture may comprise 75% or more I frame data. Thusthe spatial data is prioritized. Spatial slices for an I frame areconsidered important and are duplicated, and certain B and P framespatial slices may be duplicated. For B and P frames, if a prescribednumber of slices are found to be intracoded (eg., based on adaptiveweighting factors as a function of picture complexity), the slice isconsidered to be spatial.

VLC spatial and non-spatial data outputs of compressor 2214 are appliedto respective inputs of a tape packet generator 2216, another input ofwhich receives variable length coded audio representative words from anaudio output of AD-HDTV decoder 2210. FIG. 24 is a generalrepresentation of the tape packet format. The tape packet headercontains fields indicating a time stamp within the original datastream(eg., indicating hours, minutes, seconds and frame number of associateddata), the slice number of the data (eg., slice 50, I frame), andcontinuation data, a data section containing specific slice dataincluding starting bit and ending bit information for example, and anerror correction section containing FEC and CRC error detection andcorrection information, for example.

Continuing with FIG. 22, a tape packet datastream from unit 2216 isrecorded via tape transport and servo unit 2218, which includesrecording and playback mechanisms and electronics. An output signal isapplied via a converter 2230 to an HDTV input selector as discussed inconnection with FIG. 10, and also to a unit 2234 which eliminatesduplicte HP cells generated by unit 2215. An output signal from unit2234 is provided to an audio input of a decoder in the HDTV receiver.The output signal from unit 2234 is processed by a unit 2238 whichconverts the datastream from tape packets back to MPEG words andvariable length decodes these words. MPEG words from an output of unit2238 are applied to an MPEG-level video input of an HDTV receiverdecoder.

During playback/reproduction, the data recovered from the tape is in theform of tape packets containing variable length coded MPEG words.Merging the duplicated data into a useful MPEG datastream is facilitatedby the information contained in the tape packet headers, indicating atime stamp within the original datastream, the slice number of the dataand continuation data. As each packet is read by unit 2238, the data isconverted from variable length code to words and is stored in memory(eg., unit 2332 in FIG. 23). An index (eg., unit 2324 in FIG. 23) isfilled with packet header information. A computer controller (unit 2322in FIG. 23) decides which blocks of MPEG words are to be delivered toform the output MPEG word datastream.

The basic elements of an MPEG-level tape machine are shown in FIG. 23.Elements 2310, 2312, 2314, 2316, 2318, 2320, 2322, 2324, 2330, 2332 and2340 are similar in operation to elements 1210, 1212, 1214, 1216, 1218,1220, 1222, 1224, 1230, 1232 and 1240 in FIG. 12, respectively. Unit2311 corresponds to units 2211 and 2212 in FIG. 22. The elastic bufferin unit 2311 is resilient enough to allow a reasonable rate of variablelength coding. Timing cell data are translated to a local frame timingsignal. Variable length coder 2335 (eg., a ROM) compresses the MPEGbitstream for storage efficiency, using standard MPEG code tables.Variable length decoder 2326 subsequently undoes this coding. Decoder2326 may be implemented by the decoder (VLD) in unit 2311 sincerecording and playback are not simultaneous.

Memory 2332 stores a GOP worth of MPEG codewords. Analysis processor2330 writes data while output processor 2340 reads data. MPEG wordanalysis processor 2330 generates read/write addresses for word memory2332. During recording, processor 2330 looks for Slice Start codewordsin the wordstream, counts all spatial macroblocks, allocates duplicateslices, and delivers duplicate slices for recording. During playback,processor 2330 fills word memory 2332, scans tape packet headers, andcreates a slice index entry for each cell Slice index memory 2324receives data from analysis processor 2330 while processor 2330 isfilling word memory 2332. This memory is seen by computer-controller2322.

Output processor 2340 generates read addresses for memory 2332 duringplayback, based upon the contents of slice index memory 2324 and indicesgenerated by controller 2322. Controller 2322 loads a FIFO memory inoutput processor 2340 with address information and functional commands,and in the playback mode controller 2322 provides slice indices tooutput processor 2340. Output processor 2340 generates MPEG streamlevels above slice level and an output codeword stream including a framesynchronization signal.

The operation of the MPEG-level system at slow features speeds isgenerally similar to that previously described in connection with thecell-level system. Briefly, a GOP of data is held in word memory 2332.When the user (or a timer) calls for the next frame, it is produced fromthe memory if possible. If such data is not in memory, the tape iscaused to move to acquire the needed GOP data. Data to be output to thereceiver is repetitively played from memory. With this approach, framesynchronization is accomplished with a locally developed signaldelivered at the start of the data for each frame. The words for anoutput frame are all delivered within the frame time.

For fast features operation, the word memory is filled with data. Sliceindex 2324 contains information bits indicating if an entire slice wasacquired (and if so, which slice), if the start of a slice was acquired,or if only the end of a slice was acquired. Controller 2322 rearrangesthe slices to form frames based upon an examination of the contents ofindex 2324. Valid slices are delivered by output processor 2340 undercontrol of controller 2322.

The operations performed by the tape machine in the special featuresmode can be enhanced and assisted by the use of “feature (control)bits.” These bits may be advantageously incorporated in the headers ofthe data cells of the AD-HDTV system, for example. These bits may beresponded to at the television receiver to perform a given featurefunction by operations performed largely or entirely at the receiver,rather than at the tape machine. The AD-HDTV data format, as describedpreviously, includes timing cells. These timing cells indicate to thedecoder when to decode a specific frame. The timing cells contain thetemporal reference that indicates which frame is to be decoded at thetime that the timing cell arrives. There is room in the timing cell forextra information. It is in the timing cell that the extra feature bitsare inserted. As the decoder requires a timing cell to decode a picture,the feature bits are available to the decoder when the picture is to beprocessed. The control computer 1222 in FIG. 12, in response to usercontrols, is aware of which modes the tape machine is operating in. Thecontrol computer loads a register in the Cell Output Processor, 1240with the values of the feature bits to be inserted in a known positionin the timing cells to be produced.

A bit field may be introduced indicating and instructing a receiver to“Show the Last Frame Again.” In some AD-HDTV receivers the last frame isthe past (I or P) anchor frame. In other receivers, the last frame isthe last frame displayed. Another bit may represent an instruction to“Decode But Don't Play.” In some receivers this bit may have the effectof leaving the last frame displayed in the output buffer whilegenerating new frames. A further bit may represent an instruction to“Accept This Frame.” This instruction may be intended to accept a givenframe even if it is out of an expected temporal sequence. This actionwould begin at the next MPEG Picture Start Code (which precedes a frame)temporal reference in the datastream that matches this timing celltemporal reference. The combination of this bit and the “Show Last FrameAgain” bit may be used by a receiver to generate a new frozen frame.Another bit, “Ignore Input Data,” may be used to ignore input video datafor a given frame, leaving the current state unchanged (anchor frames,motion vectors, error concealment memory, etc.), and to disablesequential checks for data consistency. Other features bits mayrepresent instructions to permit normal audio reproduction, to mute theaudio completely, or to mute the audio selectively when there areexcessive data discontinuities to avoid unpleasant sounds.

In a slow-speed playback features mode, the tape transport mechanism maybe required to exhibit shuttle-like movements, passing over certain tapetracks before data is needed. The playback electronics include buffermemory large enough to hold a GOP of input data and a frame of outputdata for output. This is not an unreasonable amount of storage as thedata is compressed at this point. Appropriately controlled tapeacceleration can be achieved with currently available tape transportsand controllers. An exemplary sequence of operations follows.

With the tape moving forward at normal speed, the cell buffer memorycycles through its capacity (memory addresses), storing the current GOPas it is written to the buffer. The tape machine features controller(eg., unit 1222 in FIG. 12) is aware of the current position in thedatastream of tape packets, and of the corresponding current position inthe input AD-HDTV datastream. This information is conveyed by GOP andframe number, picture header temporal reference for the MPEG bitstream,timing cell temporal reference information from the AD-HDTV datastream,and sequence counters for all service types. All such information iscontained in the headers inspected by the features controller.

When the viewer activates a “Freeze” tape control, the tape mechanismcontinues to read through the current GOP, filling the buffer memorywith an entire GOP worth of cells. The tape controller locates the GOPboundary in term of cells by scanning HP video cell headers for frametype 0 while buffering LP data in memory, and noting the associatedframe number. The stored LP cell headers are then scanned for frame type0 with a matching frame number. The associated current timing celltemporal reference (“linearized” to indicate the established framedisplay order) determines the frozen frame. Other timing cells in theheader are replaced by the features controller, with timing cellscontaining feature bits indicating that the following functions are tobe performed: Freeze frame, Ignore other inputs, and Mute audio. Thefeatures controller replaces further video cells with null cells whilethe Freeze frame feature persists. The insertion of null cells into thedatastream during the freeze frame mode advantageously avoids the needto stop and start the datastream, thereby avoiding timing andsynchronizing problems. In the replacement null cells, the associatedService Type Sequence Number in the header is generated sequentially bythe features controller to validate the datastream with the replacementcells. This Sequence Number is used in the AD-HDTV system, for example,to aid in discovering lost or erroneously positioned packets, orinterchanged service types.

The tape drive transport mechanism, having read to the end of the GOP,gently slows down the tape, backs up slightly, and prepares to resumereading the tape packets for the next GOP. At this point the user isstill seeing the frozen frame display. At this time likely userinstructions include Next Frame, Previous Frame, or Play.

If the user requests the next frame, the following steps occur. If thenext frame is not in memory, the tape moves forward. The first framewritten into buffer memory is the I frame at the beginning of the nextGOP, after which the following B and P frames are written to the memorywhereby the buffer contains the entire new GOP. Video service type cellsfor the next frame are transmitted in order to the receiver. Again,video cells not within the next frame are replaced by null cells. Thetiming cell corresponding to the next frame may or may not be in thecell buffer. If the features controller senses that it is in the buffer,it is altered by the controller to match the artificial timing andsequencing being generated by the tape machine. If it is not in thebuffer, the (timing flywheel in the) tape machine causes a timing cellto be generated. This timing cell contains an “Accept This Frame”features bit and a “Show Last Frame Again” features bit, which cause thereceiver to update the displayed picture by showing the last frame.

If the user requests the previous frame, the following steps occur. Thefeatures controller knows which anchor frames are stored in thereceiver. If the requested previous frame can be generated from theexisting anchor frames stored in the receiver and from the input datafor the requested frame (ie., the requested frame is a B frame followinga P frame in transmission order), the cells to update the receiverdisplay are produced using the procedure described in the precedingparagraph. If the requested frame is an I frame, a P frame, or a B framefollowing an I frame, the tape is caused to reverse and play the entirepreceding GOP.

The table shown in FIG. 20 indicates the frame dependencies for steppingthe tape forward and backward. In this table, the “transmission order”row indicates what is recorded on tape as appearing at the output of thevideo tape machine. Assume that the GOP consisting of frames 9 through14 is the GOP in buffer memory. The “displayed frame” row indicateswhich frame could be displayed at the time the transmitted frame isavailable. B frames are displayable when they arrive, and anchor frames(I and P frames) are displayable three frames after they arrive.

In the following examples assume that, when the tape is stepped forward,the GOP consisting of frame 9 through frame 14 has just been read fromthe tape. To display frame 6, frame 6 alone is required, and it isalready in the receiver's anchor frame memory. The receiver alwaysstores two anchor frames to be able to create a B frame. Frame 9 isdelivered and frame 6 is displayed. One more forward step requires thedata for frame 7 and for anchor frames 6 and 9, which are available inthe receiver's frame memory.

When stepping backward, assume that the same GOP resides in cell buffermemory (unit 1232 in FIG. 12). The first frame to be displayed is frame14. The previously displayed frame was frame 15, so frame 15 is assumedto be in receiver memory. Frame 14 requires data from anchor frame 12,and frame 12 requires data from anchor frame 9. The entire GOP must beoutput to the receiver to prepare the receiver to display frame 14.

Stepping back to frame 13 and then to frame 12 is less difficult, sincethe required anchor frames are already in receiver memory. Frame 11requires replay of the GOP to put frame 9 into the receiver. Frames 10and 9 follow easily. Stepping back to frame 8 initiates a cascade. Frame8 requires anchor frame 6, which isn't in receiver memory, and frame 6is not in the current GOP in the tape machine's cell buffer memory.Frame 6 requires frame 3, which requires frame 0. Thus the entireprevious GOP (frames 0-8) must be read from the tape. Accordingly,frames 0 and 3 are delivered to the receiver as anchor frames. Frame 6is then delivered to the receiver, with frames 3 and 6 now being anchorframes. The GOP containing frames 9-17 is then re-acquired by goingforward with the tape and storing it in the cell memory, and frame 9 isdelivered to the receiver. Frames 6 and 9 are now the anchor frames,whereby frame 8 may be generated and delivered to the receiver.

The time to respond to a user requested step may vary, depending on thetype of frame which is requested. For some frames, for which informationis available in buffer memory, a new picture can be generated andpresented almost immediately, eg., in approximately 33 milliseconds. Thecascade example described in the previous paragraph takes considerablymore time, since that example involved repositioning the tape to thebeginning of the previous GOP (100 ms.), reading the previous GOP whiledelivering frames 0, 3 and 6 (300 ms.), reading frames 9, 7 and 8 (½ GOPtime, 150 ms.), and presentation time (33 ms.), for a total time ofabout 600 ms. This is not unreasonably long for a user response time.

Other slow-speed user features, such as slow motion and reverse, can betreated as a predictable series of still (“freeze”) frame steps. Thesesteps are advantageously undertaken at the receiver in response toinformation, such as the feature bits described previously, provided tothe receiver by the tape machine. Thus, for example, it is the receiverwhich generates a freeze-frame display using its internal memory andHDTV processing/decoding circuits in response to a control bit providedby the tape machine in response to a user control request. Slow ratescan be obtained by stepping and then repeating frames as often asnecessary to maintain a smooth rate.

The previously mentioned AD-HDTV system, sometimes referred to as theADTV system, has been submitted to the FCC/ACATS for on-going testingand analysis by the Advanced Television Test Center (ATTC).

What is claimed is:
 1. A video recording/reproducing (VRR) system forprocessing a digital enhanced definition television signal, comprisinginput means for receiving a digital datastream containing videoinformation; recording/reproducing means responsive to said datastream;output means responsive to a transduced signal from saidrecording/reproducing means for conveying information includingtransduced video information to a video device suitable for processingand displaying television-type information, when present; featurecontrol means, responsive to user input control, for generating VRRvideo feature control data to determine the operation of said videodevice such that, in response to said feature control data beingprovided from said VRR system to said video device when present, atleast one VRR video feature function is performed in whole or in part bysaid video device; and means for conveying said feature control data tosaid output means.
 2. A system according to claim 1, wherein saidfeature control data represents an instruction to perform at least oneof the following functions: (a) repeat the display of a video imageframe to produce a freeze-frame image; (b) decode but not display avideo image frame; (c) accept an out-of-sequence video image frame; (d)ignore predetermined data for a given video image frame; and (e) muteaudio information.
 3. A video recording/reproducing (VRR) system forprocessing a digital enhanced definition television signal, said systemexhibiting special features operation in accordance with a methodcomprising first and second operating modes including the steps of, fora first mode: (a) receiving a digital video information datastreamcomprising an input arrangement of video data; (b) altering said dataarrangement so that prescribed video data, when recorded at a givenspeed, appears in predetermined regions of tape tracks as a function ofa tape scanning path a recording/reproducing head of arecording/reproducing transducing device is expected to travel at aspeed greater than said given speed in a special features operatingmode; (c) recording said datastream with said altered data arrangement;and (d) conveying information including transduced information from saidrecording/reproducing device to a video device suitable for processingand displaying television-type information, when present; and a secondmode including the steps of (e) generating VRR video feature controldata in response to user input control to control the operation of saidvideo device such that, in response to said feature control data beingprovided from said VRR system to said video device when present, atleast one VRR video feature function is performed in whole or in part bysaid video device; and (f) conveying said feature control data to saidvideo device when present.
 4. A method according to claim 3, wherein insaid first mode said given speed is a normal recording speed; said step(b) special features operating mode is a fast search mode; saiddatastream comprises MPEG coded image representative informationincluding intracoded “I” picture frames; said prescribed video data isintracoded “I” frame information; said feature performed in step (e) isa freeze-frame feature; and said video device is a television signalreceiver.
 5. A video recording/reproducing (VRR) system for processing adigital enhanced definition television signal, said system exhibitingspecial features operation in accordance with a method comprising thesteps of: (a) generating VRR video feature control data in response touser input control to control the operation of a video device suitablefor processing and displaying television-type information such that, inresponse to said feature control data being provided from said VRRsystem to said video device when present, at least one VRR video featurefunction is performed in whole or in part by said video device; and (b)conveying said feature control data to said video device when present;wherein generating step (a) comprises the step of generating a freezeframe display using local memory associated with said video device.